[PATCH] virtio_ring: use u32 for virtio_max_dma_size

[zhenwei pi]

Both split ring and packed ring use 32bits to describe the length of
a descriptor: see struct vring_desc and struct vring_packed_desc.
This means the max segment size supported by virtio is U32_MAX.

An example of virtio_max_dma_size in virtio_blk.c:
  u32 v, max_size;

  max_size = virtio_max_dma_size(vdev);  -> implicit convert
  err = virtio_cread_feature(vdev, VIRTIO_BLK_F_SIZE_MAX,
                             struct virtio_blk_config, size_max, &v);
  max_size = min(max_size, v);

There is a risk during implicit convert here, once virtio_max_dma_size
returns 4G, max_size becomes 0.

Fixes: e6d6dd6c875e ("virtio: Introduce virtio_max_dma_size()")
Cc: Joerg Roedel <jroedel@xxxxxxx>
Signed-off-by: zhenwei pi <pizhenwei@xxxxxxxxxxxxx>
---
 drivers/virtio/virtio_ring.c | 12 ++++++++----
 include/linux/virtio.h       |  2 +-
 2 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index c5310eaf8b46..55cfecf030a1 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -289,12 +289,16 @@ static bool vring_use_dma_api(const struct virtio_device *vdev)
 	return false;
 }
 
-size_t virtio_max_dma_size(const struct virtio_device *vdev)
+u32 virtio_max_dma_size(const struct virtio_device *vdev)
 {
-	size_t max_segment_size = SIZE_MAX;
+	u32 max_segment_size = U32_MAX;
 
-	if (vring_use_dma_api(vdev))
-		max_segment_size = dma_max_mapping_size(vdev->dev.parent);
+	if (vring_use_dma_api(vdev)) {
+		size_t max_dma_size = dma_max_mapping_size(vdev->dev.parent);
+
+		if (max_dma_size < max_segment_size)
+			max_segment_size = max_dma_size;
+	}
 
 	return max_segment_size;
 }
diff --git a/include/linux/virtio.h b/include/linux/virtio.h
index b93238db94e3..1a605f408329 100644
--- a/include/linux/virtio.h
+++ b/include/linux/virtio.h
@@ -147,7 +147,7 @@ int virtio_device_restore(struct virtio_device *dev);
 #endif
 void virtio_reset_device(struct virtio_device *dev);
 
-size_t virtio_max_dma_size(const struct virtio_device *vdev);
+u32 virtio_max_dma_size(const struct virtio_device *vdev);
 
 #define virtio_device_for_each_vq(vdev, vq) \
 	list_for_each_entry(vq, &vdev->vqs, list)
-- 
2.20.1